Advantages of parallel imaging in conjunction with hyperpolarized helium - A new approach to MRI of the lung

Hyperpolarized helium (³He) gas MRI has the potential to assess pulmonary function. The non-equilibrium state of hyperpolarized ³He results in the continual depletion of the signal level over the course of excitations. Under non-equilibrium conditions the relationship between the signal-to-noise ratio (SNR) and the number of excitations significantly deviates from that established in the equilibrium state. In many circumstances the SNR increases or remains the same when the number of data acquisitions decreases. This provides a unique opportunity for performing parallel MRI in such a way that both the temporal and spatial resolution will increase without the conventional decrease in the SNR. In this study an analytical relationship between the SNR and the number of excitations for any flip angle was developed. Second, the point-spread function (PSF) was utilized to quantitatively demonstrate the unconventional SNR behavior for parallel imaging in hyperpolarized gas MRI. Third, a 24-channel (24ch) receive and two-channel (2ch) transmit phased-array system was developed to experimentally prove the theoretical predictions with ³He MRI. The in vivo experimental results prove that significant temporal resolution can be gained without the usual SNR loss in an equilibrium system, and that the entire lung can be scanned within one breath-hold (∼13 s) by applying parallel imaging to 3D data acquisition.